1. Field of the Invention
The present invention relates to metallization techniques which utilize a plasma to treat a surface to which metal will be applied. Particularly, the present invention relates to plasma treatment of the surface of a semiconductor integrated circuit and the subsequent metallization of that surface. More particularly, the technique of the present invention is useful for placing refractory metals, refractory metal nitrides, and refractory metal silicon nitrides on a semiconductor die. A preferred metal for use in the technique of the present invention is tungsten.
2. State of the Art
Thin layers of refractory metals are desired for use in integrated circuits for several purposes, including, without limitation, as low resistance gate interconnections in polysilicon gate regions of field-effect transistors, to form a Schottky-barrier, to form ohmic contacts on silicon, as low resistance vias, and others. As line widths in very large scale integrated circuits (VLSI) decrease, the use of refractory metals in such circuits becomes increasingly desirable.
Traditionally, refractory metals for VLSI applications have been deposited by sputtering, evaporation, and chemical vapor deposition (CVD) onto the active surface of a wafer, or semiconductor substrate, including without limitation silicon, gallium arsenide, silicon on sapphire (SOS), silicon on insulator (SOI), silicon on glass (SOG), and other wafers known in the art. Although it is possible to sputter almost any material, including without limitation pure refractory metals and refractory metal silicides, sputtering machines tend to be complicated and tend to require considerable maintenance. Sputtering also tends to deposit refractory metals and their silicides in an inconsistent manner which leaves pinholes and other discontinuities in the deposited layer (i. e., sputtered films are not conformal).
Evaporation techniques for depositing refractory metals have many of the same deficiencies as sputtering techniques. Specifically, evaporative deposition of refractory metals is often a complex process. Many techniques which deposit refractory metals by evaporation also provide poor refractory metal coverage.
Relative to sputtering and evaporation techniques, chemical vapor deposition and low pressure chemical vapor deposition (LPCVD) of refractory metals often provide good coverage with reduced system complexity. However, many CVD techniques are somewhat undesirable in that the layers of refractory metals formed thereby are inadequate for some uses in integrated circuits. Many CVD techniques also deposit inconsistent, discontinuous layers of refractory metal. It is also difficult to reproduce the thickness of the refractory metal films produced by some existing CVD techniques.
U.S. Pat. No. 5,540,607, issued to Tsao (the xe2x80x9c""607 Patentxe2x80x9d), describes a method for treating the surface of a field-effect transistor or a Schottky barrier diode. The technique of the ""607 Patent utilizes a plasma etch such as CF4+4% O2 to shape doped polysilicon regions. According to the method of the ""607 Patent, the polysilicon surface is then treated with a low power argon plasma to enhance nucleation sites on that surface. A layer of tungsten or molybdenum is then selectively deposited on the treated surface by CVD techniques.
Although the method of the ""607 Patent enhances the nucleation sites on a polysilicon or silicon surface, the refractory metal which is subsequently deposited by the reaction of WF6 with silicon may cause wormholes, which decreases the selectivity of refractory metal deposition. The refractory metal layer may also have an inconsistent thickness (i.e., poor uniformity).
U.S. Pat. No. 5,618,382, issued to Mintz et al. (The xe2x80x9c""382 Patentxe2x80x9d), describes an apparatus which, among other things, employs a plasma to etch a substrate. The apparatus operates at high frequencies (greater than 13.56 MHz), preferably in the range of 30 to 200 MHz, depending upon the process to be performed on the wafer. The purpose of the apparatus disclosed in that patent is to increase the rate of plasma processing of semiconductor wafers without significant damage thereto. The ""382 Patent does not disclose use of the apparatus to deposit refractory metals onto a semiconductor die, nor does the ""382 Patent disclose improved adherence of a refractory metal layer to a semiconductor die following plasma surface etching.
In integrated circuit manufacturing, blanket deposition of tungsten is typically favored over selective deposition of the same due to difficulties in controlling selectivity loss. Blanket deposition inherently requires that the tungsten be deposited onto both silicon/polysilicon and oxide surfaces. Typically, prior to the blanket deposition of tungsten, multiple layers including titanium (Ti) and titanium nitride (TiN) are deposited onto the silicon and/or polysilicon surfaces prior to the deposition of more desirable refractory metals such as tungsten or molybdenum. A first layer, such as a titanium film, which adheres well to silicon, polysilicon, or the like, is deposited directly onto such surfaces. A second layer, such as titanium nitride, which adheres well to both the first layer and to the desired refractory metal, is then deposited onto the first layer. Next, the desired refractory metal, such as titanium or molybdenum, is deposited onto the second layer. The titanium/titanium nitride layers also lower contact resistance and act as a diffusion barrier to prevent fluorine-containing species from attacking the underlying layers during subsequent deposition steps.
Although the use of such layers improves the adhesion of tungsten or other refractory metals to a die, materials such as titanium nitride oxidize very readily under ambient conditions. As those of ordinary skill in the relevant art are aware, refractory metals do not adhere well to oxides. Thus, an oxidized surface of the second layer hinders the ability of the desirable refractory metal to form a contiguous film which will adequately adhere to the second layer (i.e., causes less uniform nucleation and increases the length of time that is required to form a contiguous film).
A method is needed in the art for decreasing or eliminating oxidation on a substrate prior to metallization, so as to improve the adhesion of refractory metals, refractory metal nitrides and refractory metal silicon nitrides to the substrate. A method for producing a more contiguous, uniform layer of refractory metal and in a reduced period of time is also needed. Further, a method is needed which reduces the incidence of spontaneous fluorine attack of the substrate material.
The method of the present invention addresses each of the foregoing needs.
As used herein, the term xe2x80x9crefractory metalsxe2x80x9d means refractory metals, refractory metal nitrides, refractory metal silicon nitrides, and other molecules and materials which include refractory metal atoms. The terms xe2x80x9cwaferxe2x80x9d and xe2x80x9csemiconductor substratexe2x80x9d, as used herein, refer to semiconductor wafers formed from silicon, polysilicon, gallium arsenide, silicon on glass (SOG), silicon on insulator (SOI), silicon on sapphire (SOS) and others known in the art.
According to the method of the present invention, one or preferably more layers of base material are adhered to a semiconductor substrate material prior to deposition of a refractory metal layer thereover. Each of the layers of base material between the substrate and the refractory metal adhere well to both of the adjacent layers. Preferably, the upper, exposed base layer includes a material to which a desired refractory metal layer adheres well. A plasma is then generated over the upper base layer to reduce the level of oxidation thereon and to facilitate the subsequent deposition of a refractory metal, such as tungsten. Preferably, the plasma gas includes a mixture of argon, hydrogen and nitrogen. The impact of argon against the titanium nitride surface may physically remove oxygen atoms from the surface or it may energize the oxygen atoms, causing them to react with the hydrogen radicals in the plasma. A layer of refractory metal is then applied to the upper base layer. Preferably, the refractory metal is deposited onto the substrate in situ by CVD techniques immediately following the removal of oxygen from the base layer surface over the substrate.
The present invention includes a method for treating an exposed, upper base layer onto which a refractory metal such as tungsten, molybdenum, or another refractory metal or metal alloy is to be applied. An argon-hydrogen-nitrogen plasma is generated over the base layer. Argon molecules in the plasma collide with the exposed surface of the upper base layer and remove oxygen atoms therefrom. Hydrogen and nitrogen radicals coat or xe2x80x9cstuffxe2x80x9d the upper base layer surface to prevent further oxidation thereof.
The present invention also includes a method for reducing oxidation on the layer upon which the refractory metal layer is to be deposited, further improving the adherence of the refractory metal to that layer.
The method of the present invention is useful for forming conductive structures in semiconductor devices, including without limitation electrical contacts, such as bond pads, via fills, gates, and local and global interconnections.
Advantageously, the present invention provides a method to form a contiguous, consistent layer of refractory metal film on the exposed surface of a substrate. The method of the present invention includes a first chemical vapor deposition (CVD) step which facilitates the generation of nucleation sites, which have the thickness and confluency to form a contiguous layer on the substrate, and provide improved adhesion of the subsequently deposited refractory metal layer thereto. The preferred method also includes a subsequent CVD step, wherein a second chemical reaction facilitates growth of nucleation centers together and consistent thickness of the film in order to increase step coverage and reduce contact resistance. Thus, the method of this invention deposits a contiguous, consistent layer of refractory metal on the substrate. The present invention also reduces the time it takes to deposit a contiguous layer of refractory metal onto the surface of a die relative to refractory metal deposition processes in the prior art.
The incidence of spontaneous fluorine etching of the substrate is also reduced by the method of the present invention. This is accomplished by excluding fluorine-containing species from the plasma, which are known by those of ordinary skill in the relevant art to etch silicon, polysilicon, silicon dioxide, and other materials, from the de-oxidizing plasma. Instead, hydrogen radicals from the plasma create an excess of hydrogen, which adheres to the upper base layer surface following de-oxidation of that layer. Thus, the fluorine atoms of the deposition reaction are believed to be more likely to react with the hydrogen than with the substrate.